Clean water is essential to human health. It is also a critical feedstock in a variety of key industries including the electronics, pharmaceutical and food industries. Treatment of groundwater, lakes and reservoirs is often required to make water safe for human consumption. For wastewater, treatment is necessary to remove harmful pollutants from domestic and industrial liquid waste so that it is safe to return to the environment. Current water treatment systems are generally large, centralized systems that comprise a number of steps, including treatment with anaerobic organisms, oxidizers, chlorine, and flocculants.
Because of their inherent flexibility, decentralized water treatment systems could provide more robust and cost effective means for dealing with (i) declining sources of freshwater, (ii) more stringent water quality standards and (iii) chemical and biological threats to local water supplies. It has been proposed that distributed optimal technology networks (DOT-NET) are an alternative to the large, centralized water treatment plants. The DOT-NET concept is predicated upon the distribution and strategic placement of relatively small and highly efficient treatment systems at specific locations in existing water supply networks. Filtration processes that remove specific contaminants are a key aspect of decentralized water treatment systems.
A number of water filtration processes have designed to remove organic compounds and metal ions from contaminated wastes been described in the literature. Two such processes are micellar-enhanced ultrafiltration (MEUF) (Scamehorn and Harwell, (1988) In Surfactant Based Separation Processes, Surfactant Science Series, Vol 33, Marcel Dekker, New York, Dunn et al., (1989) Coll. Surf 35:49, Baek et al., (2004) J. Haz. Mater. 1081:19, Richardson et al., (1999) J. Appl. Polym. Sci. 4:2290) and polymer supported ultrafiltration (PSUF) (Spivakov et al., (1985) Nature 315:313, Geckeler et al., (1996) Envir. Sci. Technol, 30:725, Muslehiddinoglu et al., (1998) J. Memb. Sci, 140:251, Juang et al., (1993) J. Membrane Sci. 82:163). In a typical MEUF process, a surfactant is added to polluted water. The aqueous solution is then passed through an ultrafiltration membrane with pore sizes smaller than those of the organic/metal ion laden micelles. In PEUF, a water-soluble linear polymer with strong binding affinity for the target metal ions is added to contaminated water. The resulting solution is passed through an ultrafiltration membrane (UF) with pore sizes smaller than those of the metal ion-polymer complexes.
MEUF is based on the use of non-covalently bonded micelles to extract organic solutes and/or bind metal ions. Micelles are dynamic and flexible structures with finite lifetime. Because of this, their size, shape, organic solubilization capacity, metal ion binding capacity and retention by UF membranes are very sensitive to surfactant concentration and solution physical-chemical conditions (e.g., pH, temperature, ionic strength, etc). Although the use of micellar solutions of height molecular weight block ABA copolymer of PEO-PPO-PEO surfactants could reduce surfactant losses to a certain extent (Richardson et al., (1999) J. Appl. Polym. Sci. 4:2290), the leakage of surfactant monomers remains a major problem in water treatment by MEUF.
In most cases, the surfactant solutions in MEUF processes are not very selective and have relatively low organic solute and metal ion binding capacity. For the most part, they solubilize organic solutes through partitioning in their hydrophobic core and bind metal ions through electrostatic interactions with their charged head-groups. Moreover, the development of surfactant solutions with redox, catalytic and biocidal activity remains a major challenge. Thus, MEUF has remained for the most part a separation process with limited practical applications.
The PSUF process has been primarily designed and evaluated to remove metal ions from contaminated wastewater streams. PSUF uses high molar mass linear polymers such as EDTA and macrocycles with amine groups (e.g., cyclams) that typically bind only one metal ion per molecule. While the components of a MEUF filtration system are somewhat adaptable to different functional groups, the PSUF process is not readily functionalizable, and neither MEUF nor PSUF have been shown to be capable of catalytic reactions. Due to the ongoing demand for clean water and the limitations of the current methods, there is a significant need in the art for a new water filtration process with a higher capacity for binding contaminants, as well as features that enable it to be scalable, flexible, and configurable to suit a variety of different water purification needs.